This invention relates to a process for granulating metal sulfide powders, and more specifically to a process for forming the powder of a metal sulfide, such as molybdenum disulfide or tungsten disulfide, into granules within desired limits of particle size.
Self-lubricating parts such as those of oilless bearings have usually been fabricated by mixing the powder of copper, tin, or other metal, as the base, with the powder of a solid lubricant, pressing the mixture in dies to produce compacted shapes, and sintering the shapes. While the solid lubricant powder is available in many different kinds, the powders of metal sulfides having layer crystalline structures, such as molybdenum disulfide and tungsten disulfide, may be cited as typical examples. It is because of their ideal layer structures that such two disulfides are particularly useful as solid lubricants. Such a solid lubricant powder, if directly added to the powder of the base metal powder, would not give a good self-lubricating product, for a number of reasons including the infeasibility of achieving uniform mixing and the low strength attained on sintering. As an alternative, therefore, composite powders consisting of solid lubricant particles coated with copper, nickel, or other metal have frequently been employed.
Metal sulfide powders commercially available mostly take the form of fine particles that pass the 400-mesh sieve. Responsible for this fineness are limitations in the processes for preparation. In the case of natural products, the powders are purified by flotation, and synthesized products are subjected to a process for elimination of by-products, and in either process only very fine particles will result. When a metal sulfide powder consisting of such fines is added as a solid lubricant to a base metal powder and the mixture is compacted with pressure, the addition in a sufficient amount for producing an adequate lubrication effect would cause a sharp decrease in the strength of the sintered compact. For example, a base metal powder of a copper-tin system, with the addition of 3-5% by weight of the metal sulfide powder, will give a sintered product with little improvement in the self-lubricating capability. The addition of more than 8-10% by weight for the first time will produce a lubrication effect but the strength of the product will be seriously low. For this reason it is desirable to granulate the solid lubricant, prior to the addition, to a particle size ranging from 4 to 400 mesh, for example.
As mentioned above, coating of metal sulfide particles with a second metal is a modern practice for improving the lubricating ability of the sintered product without the strength penalty. In the preparation of such a composite powder, the yield of coating varies extremely with the particle size of the particular metal sulfide powder to be coated. To cite one example, the production yield of a copper-coated molybdenum disulfide powder by the addition of a copper sulfate solution and iron powder to molybdenum disulfide powder is at most 80% when the particle size of the disulfide powder is finer than 400 mesh, whereas the yield soars to over 99% with disulfide particles in the range from -4 to +400 mesh. When using a metal sulfide powder in the composite form, therefore, it is more effective to coat the sulfide particles granulated in advance than coating the powder as such.
Another justification for granulation is the fact that the individual particles of commercially available metal sulfide powders are flat shaped with many projections. If the particles are directly coated, the second metal will preferentially deposit on the projections, making the coated film uneven, or causing rounding off of the particles during washing, drying and other processing, thereby producing finer particles anew. Even when the metal coating is not done, problems, such as formation of fines, un-uniformity of mixing, and low sintered strength, can arise. Thus, prior granulation of such flat particles having many projections is beneficial in that the projections are eliminated and the individual particles are made spherical.
As will be appreciated from the foregoing description, it is necessary to granulate and size the fine metal sulfide powder of commerce to a desired range of particle size before its addition as a solid lubricant, either directly as it is or in the form of composite powder, to the base metal powder.
No attempt has hitherto been made to granulate metal sulfide powders. The object of granulation herein contemplated is to convert very fine powders of less than 400 mesh into granules sized within the range from 5 to 400 mesh. Unlike pelletization or the like that does not require closeness in size, the granulation in accordance with the invention calls for careful considerations. Granulation of fine powdery materials other than metal sulfides, e.g., carbon black, have been in practice. The conventional methods of granulation are generally classified into wet processes which involve addition of water and dry processes in which the material powder is compressed, compacted, and broken to pieces. None of the processes have, however, proved satisfactory, because of a cumbersome after treatment, large proportion of excessively coarse particles, uneven particle size, and/or low yield of product. With these shortcomings, the ordinary processes do not give favorable result when applied to the granulation of metal sulfide powders.